1. Field of the Invention
The present invention relates to electrodes for use in resistance spot welding, and in particular to a composite electrode incorporating specially configured separate strength and insulation members which substantially increase the useful life of the electrode by mitigating the effects of the high contact pressures and high current densities encountered.
2. Description of the Prior Art
The problems associated with maintaining good electrode structural integrity and operating performance during resistance spot welding have received a good deal of attention over the years. This is so because of the critical role electrodes play in assuring consistant weld quality, and because of the severe operating requirements placed on the electrodes. Electrodes are required to: conduct the proper welding current to the workpieces; transmit the amount of compressive force to the workpiece needed to produce a satisfactory weld; and rapidly dissipate the heat from the weld zone on completion of the weld cycle. Therefore, during the welding operation, the electrodes are subject to great compressive stresses at elevated temperature, and must be regularly dressed and periodically replaced. Because the current conducted to the work piece must remain localized within a fixed area, the electrodes must resist the stresses without excessive deformation. The electrode force, in addition to forging the heated workpieces together, influences the passage of current to the localized area.
The face of a spot welding electrode contacts the workpiece directly above and/or below the point of fusion, and this small area is subjected to repeated application of high temperature and pressure. Shape and dimensions of the electrode tips are affected by mechanical wear and deformation or `mushrooming`, at a rate depending on tip material and design, operating temperature, rates of heating and cooling, and welding force. Also, the possibility of `pick-up` and alloying with the workpiece is attendant to the deformation problems--all of which tend to seriously reduce electrode life and weld quality.
This full range of electrode problem areas have been addressed by a variety of approaches, none of which have been entirely successful. As a result, there has been a continuing and serious effort to improve the service life of electrodes in daily use. The ever-increasing welding applications in which electrodes are called upon to perform manifests the need for an improved electrode capable of withstanding the effects of localized high contact pressures and high joule heating resulting from localized current densities. The composite electrode disclosed in the present invention is directed to providing such a needed device.